THICK FILM HEATED OVEN WITH LOW ENERGY CONSUMPTION
Helga Steinbrück¹, Dr. Srinivasan Sridharan², Petter Svanbom³, Orville Brown⁴, Dr. Sandrine Ringler⁵, Keith Mason⁶ and Pieter Dijkstra^{7 1}, ³, ⁵ Electrolux Major Appliances, Primary Development Cooking, D-91541, Rothenburg o.d.T, Germany
² Ferro Corporation, Posnick Center for Innovative Technology, 7500 East Pleasant Valley Road, Independence, OH 44131, USA
⁴Ferro Corporation, Electronic Materials Systems, 3900 South Clinton Avenue, South Plainfield, NJ 07080, USA
⁶Ferro Corporation, Electronic Materials Systems, 1395 Aspen Way, Vista, CA 92083, USA
⁷Ferro Corporation, Industrial Coatings Group, Van Helmontstraat 20, 3029 AB, Rotterdam, Holland

Abstract
n this work we have demonstrated the concept of a thick film heated oven, and its materials system, that pass the electrical safety requirements for stationary class I heating appliances as outlined in IEC 60335-1. This was accomplished, in part, through the development of low leakage fast-fire enamel dielectrics, compatible resistor & over glaze materials by Ferro Corporation. This was combined with the development of a new thick film heating pattern design from Electrolux Major Appliances. The resulting oven a) exhibits a uniform temperature distribution across the heated surface, b) reaches the required maximum operating temperature of 300 °C at less than 1500 W per plate, c) has a leakage current of < 0.75 mA per applied kW at 300 °C, d) has a breakdown voltage strength greater than 1250 V AC (at > 300 °C), e) exhibits an excellent life time of > 2000 ON-hours without failure, and, f) shows cooking results comparable to conventional ovens with tubular heating elements.

Introduction
A film-heated oven, wherein the cavity walls are directly heated by thick film heaters embedded onthe enamel coating to minimize the energy consumption, has been the subject of interest for the appliance industry as a whole, and Electrolux in particular. The main obstacle was achieving the desired electrical characteristics at an acceptable operating temperature of the oven without cracking in the enamel layer.
These problems arise mainly from the different materials involved and the designed shape of the oven. The materials combination, normally, comprises a thin gage low carbon steel substrate, enamels that provide electrical insulation, a resistive heating film that generates heat, and then finally, an over glaze layer to protect the resistor traces. First, conventional porcelain enamels with excessive amounts of alkali oxides in their compositions become electrically conductive for all practical purposes at low temperatures, compared to the oven operating temperatures of about 200 to 350 °C. Secondly, due to different thermal and elastic properties of the metal substrate and glass based enamel systems, thermal residual stresses develop, which can lead to cracking when these tensions are not controlled properly.
To resolve the first problem, Ferro developed new alkali free enamels, compatible resistors and
over glaze materials that fire in conventional porcelain enamel fast fire furnaces. To resolve the second problem, Electrolux investigated fabricating and optimizing (for uniform heat distribution) a Thick Film Heater (TFH) on flat steel plates. Then, the electrical and cooking performance of an oven cavity design that incorporates these TFH panels was studied. This work is presented here.

Low leakage enamels
The enamel layer in a film heated oven panel electrically separates the base (low carbon steel) metal substrate and the resistor layer. Therefore, the required electrical performance of low leakage current (LC), and high breakdown voltage (BDV) are governed by the electrical properties of the enamel layer. These include insulation resistance (IR), dielectric properties such as dielectric constant (K), loss factor, as well as thickness of, pore size and pore distribution in the enamel layer. Further, their variations with temperature determine the useful upper operating temperature of the resulting film heated oven panel.
Conventional porcelain enamels for low carbon steel substrates are, in general, based on alkali borosilicate glasses. These glasses fire at a typical peak temperature of 780-850 °C

Fig. 1 Comparison of insulation resistance of (a) a conventional porcelain enamel, with that of (b) a low leakage enamel of the current development.

(about 5 minutes above 700 °C) in a continuous fast belt furnace wherein the parts are ‘in’ and ‘out’ of the furnace in about 20 minutes. Due to excessive alkali ions in these enamels their insulation resistance quickly degrades with rise in temperature, especially in the temperature regime 200-300 °C as shown in Fig. 1 (see conventional enamel). In response to that, Ferro developed a series of alkali free glasses for enamel applications that fire in a typical porcelain enamel furnace firing conditions [1]. As shown in Fig.1, IR of these low leakage enamels is about four to five orders of magnitude higher than that of the conventional enamels in 200-350 °C temperature region. Further these electro statically coated low leakage enamels exhibit a leakage current to power ratio of about 0.12 mA/kW or less at 300 °C, for a typical enamel fired thickness of 200-250 μm [1].
Oven cavity panels, electrostatic spray process exhibited often a) uncontrolled cracking in the enamel, and b) premature BDV failures. The former was traced to enamel thickness variation as well as curvatures in the embossed cavity panels. The occurrence of premature BDV failures was traced to bigger bubbles in the enamel which were difficult to control due to the nature of the electrostatic spray process. This premature BDV failure problem gets especially severe, when the enamel thickness is reduced to reduce the warpage of the panel. To overcome these problems, alternate techniques of enamel application were undertaken. The focus was mainly on the screenprinting method for enamel application, to control the enamel thickness variation and to reduce the overall enamel thickness. Further, the oven panel design was changed to flat configuration.
Enamel compositions were modified to control: a) bubble formation due to reactions with the metal substrate, b) bubble size, and c) resistor/enamel interactions.

Screen printing pastes
Dielectrics (enamels)
Two new dielectric (enamel) screen-printing pastes, HT018 and HT051, were developed. As shown in Fig. 2, HT018 is the bonding dielectric that promotes

Fig. 2 Cross section of HT018 dielectrics on the low carbon steel substrate fired at 775 °C. Note the formation of almost bubble free interaction zone at the dielectrics/metal interface.

excellent bonding and adhesion to the low carbon steel substrate, without generating significant amount of bubbles at the interface. HT051 is the top dielectric. HT051 was designed to have excellent compatibility with both the bonding dielectric HT018 and the post fired (@630 °C) resistor paste HR81-.010 (from Ferro). Further, HT051 is also designed to have a better thermal expansion matching to the low carbon steel to reduce warpage, which becomes significant as the dielectric layer thickness increases. Although HT051 dielectric is stated to be the top dielectric, it could also be used as the bonding dielectric in place of HT018.
Fig. 3 is the cross section of HT051 dielectric printed and then fired at 780 °C, which shows very few but fine bubbles, and good bonding to the low carbon steel substrate. These microstructures passed the critical requirement of BDV >1250 V AC for 60 sec at both 25 °C and 350 °C, without failure for the dielectric thickness of about 125 μm. As we see in Fig. 4, these dielectrics exhibit very low leakage up to 400~450 °C and breakdown occurs only at temperatures greater than ~550 °C during 1250 V AC 60 sec BDV testing. On the other hand, conventional enamels fail this test even at room temperature.

Fig. 3 Cross section of HT051 dielectric on the low carbon steel substrate fired at 780 °C. Note the formation of interaction zone without any bubbles formed at the interface. Note also the excellent microstructure of dielectric with very few but fine bubbles.

Fig. 4 Observed leakage current as a function of temperature for a) conventional enamel, and b) low leakage dielectrics (enamels), when 1250 V AC was applied for 60 sec across the dielectric. The limit of leakage was set 10 mA in the Hi Pot tester.

Resistor: The lead free and cadmium free resistor paste HR81-.010 (from Ferro) used has thefollowing characteristics:
:: PTC behavior
:: Sheet resistance: 10 mΩ/sq at room temperature
:: TCR: 3100 ppm/°C
:: Firing temperature: 630-700 °C (10 min)
Over glaze: The lead free and cadmium free over glaze paste HT039 (from Ferro) was used for this work [2]. HT039 is designed to minimize any resistance shift due to any over glaze/resistor interaction either during over glaze firing (610~650 °C) or during subsequent life time testing. Fig. 5 shows the excellent compatibility between the dielectric/resistor/over glaze materials combination used for this work.

Fig. 5 Cross section of HT051 dielectric/HR81-.010 resistor/HT039 over glaze fired on the low carbon steel substrate showing the excellent compatibility between these three materials.

Fig. 6 Observed temperature distribution on the TFH plate in real time IR monitoring experiment.

Generation of the film heating pattern
A proprietary heater pattern was developed by Electrolux to get an optimized heat distribution over the whole panel’s surface of 426 mm x 386 mm. The temperature distribution due to this pattern at a given operating temperature was simulated for 400 V AC application. Then the parameters of the heater design were optimized to minimize the temperature gradient over the entire surface. Once simulated the identified pattern is applied and fired on the substrate, using the resistor paste HR81- .010. The temperature distribution in these real panels was then monitored with an infra red (IR) camera and compared to that of the simulation. In general, simulation predicts the actual performance very well. Fig. 6 shows one such observed temperature distribution on a TFH plate.

Testing
Electrical safety
The main achievement in this project is the fact that the thick film heating panels pass the electrical safety requirements for electrical household appliances: i.e. the panels pass the leakage current and the high voltage breakdown safety tests for stationary class I heating appliances, according to IEC 60335-1[3]. That means that the leakage current does not exceed a value of 0.75 mA per applied kW and no breakdown occurs during 60 sec of applied 1250 V AC voltage at operating temperature (~300 °C) [4].

Fig. 7 Observed ratio of the leakage current to applied power (mA/kW) as a function of the temperature reached on the non elemental side of the TFH panels coated with a) HT018 and b) HT051 dielectric enamels. Both panels exhibited a steady state power output of ~1500 watts.

(non elemental side of) the panel for TFH panels coated with either HT018 or HT051 dielectric as the bond coat. This figure clearly shows that the ratio of the leakage current to applied power for both dielectric enamels, is well within the safety limit of 0.75 mA per applied kW. Even though both dielectric enamels are suitable for passing the leakage current test, HT051 shows a rather increased performance, especially when applied as first layer on the substrate. According to the standard, during the testing the applied voltage has to be set at ~7 % higher than under normal conditions. The leakage current is then measured until the device reaches the steady state condition (which is at around 340 °C). As can be seen in Fig. 7, the leakage current is somewhat lower for the panel coated with the HT051 dielectric enamel, and hence better performance.
Furthermore, with the present thickness of the dielectric enamel (~200 μm), irrespective of the dielectric enamel used (HT018 or HT051), the thick film heating panels pass the high voltage break down test at operating temperatures, when the dielectric enamels are applied in particle free environment.

Heat up rate
Fig. 8 shows that the thick film heated panels reach the steady state operating temperature of 300~320 °C in about 8 ~10 minutes.

Fig. 8 Plot of time to rise to temperature for thick film heated panels coated with two different dielectric enamels.

Lifetime test
The endurance test for the film heating panels is performed according to an Electrolux internal verification standard for tubular heating elements. The test requires that minimum eight panels shall be run at operating temperature while cycling 45 min ON and 15 min OFF. The test goes on until 2000 ON-hours are reached. No failure may occur during this test period.
Nine prototypes of thick film heating panels were put on lifetime test (after having passed the electrical safety tests). Out of these two prototypes failed due to reasons other than that in lifetime testing. So the failures were not truly representative of life time testing. The remaining seven panels passed the lifetime testing. The change in resistance of these seven panels did not exceed ±5 % compared to the starting value at room temperature. Thus these panels passed the lifetime test.

Thick film heated oven
Implementation
Electrolux has developed a concept to implement the thick film heating panel as a bottom heating element in the oven. The cavity bottom is heated directly by the thick film heating panel. With this method of implementation, the prototype oven has got a low stay with its energy consumption below the threshold of 800 Wh, which is equivalent to or better than energy class A for this cavity size, according to [5].
Further, in this new oven concept, with bottom thick film heated oven panel, the maximum temperature on the cavity bottom does not exceed 300 °C, while the baking performance remains equally good (see Fig. 9). On the other hand, in conventionally heated ovens the cavity bottom reaches easily 400 °C or more. The lower the cavity temperature is, the easier it is to clean the burnt spillages, etc.

Baking results
Fig. 9 shows the cooking results for different standardized test recipes made in an oven that was run with a thick film heating panel as the bottom-heating element. This thick film heating panel is designed as previously described. It is implemented in the oven’s cavity by replacing the conventional bottom-heating element. The top heater is a non-modified conventional tubular heating element. The results show that for most of the main test recipes, the film heating device is an adequate replacement for conventional tubular heating elements.

Fig. 9 Cooking results for the standard recipes obtained with (a) conventional oven with tubular heating element, and (b) novel oven with thick film heating panel at the bottom. The results are rated from 0 to 5 where 5 is the maximum score.

Conclusions

Acknowledgements
The authors want to thank:

References
[1] S. Sridharan, et.al., “Porcelain Enamel Composition for Electronic Applications”, U.S. Patent 5,998,037, Dec 7, (1999).
[2] S. Sridharan, et.al., “Electronic Device having Lead free and Cadmium free Electronic Over glaze Applied Thereof”, U.S. Patent Pub. No. US2004/0018931A1, Jan 29, (2004).
[3] IEC 60335-1 International Standard, “Household and similar electrical appliances – Safety – Part 1: General requirements”, (2001).
[4] IEC 60335-2-6 International Standard, “Household and similar electrical appliances – Safety – Part 2-6: Particular requirements for stationary cooking ranges, hobs, ovens and similar appliances”, (2002).
[5] EN 50304 European Standard, “Electric ovens for household use – methods for measuring the energy consumption”, (2001).